You can't forget the the "rating=>" at the beginning. So it should look something like this:
Code:
replace_metadata:
rating=>M=>Adults Only
rating=>T=>Teen
rating=>K\+=>Older Kids
rating=>K=>Kids
You need to be careful with special characters such as .+?[]() etc. Since this is a regular expression you'll need to escape them with \. And just copy that section and adjust the middle part, i.e. M, T, K+, K to whatever the other site uses.